In Japanese Laid-Open Patent Publication No. 01-221816, a switch ON/OFF detecting device is disclosed, which detects whether or not a switch has been turned on, by connecting a resistor and the switch in series between a power source and ground, and determining whether or not the potential of a connection midpoint between the resistor and the switch is greater than a certain threshold.
However, with the technique disclosed in Japanese Laid-Open Patent Publication No. 01-221816, because the resistor is formed therein, it is likely for the potential of the connection midpoint to vary due to the current value. On the other hand, a leakage resistance, which is connected in parallel equivalently with the switch, occurs due to flooding or the like. However, a variance in the resistance value occurs by the amount of flooding water, or by impurities contained within the water, thus having a significant influence on the leakage current value that flows through the leakage resistance. As a result, the potential at the connection midpoint varies greatly, and setting of the threshold value is difficult to perform. For this reason, according to Japanese Laid-Open Patent Publication No. 01-221816, a large current flows for a fixed time to the switch, and by comparing the connection midpoint potential with a threshold, a correct ON/OFF state of the switch is detected. However, energy consumption increases by this amount.
The present invention has the object of providing a switch operation determining device that simplifies setting of the threshold value, while suppressing energy consumption.
A switch operation determining device of the present invention comprises a switch, and a determination circuit for determining whether or not the switch has been operated. The determination circuit further comprises a first resistor, a first diode, and a second diode, which are serially connected in this order from a power source, such that current flows to ground from the power source, and a controller that compares an anode potential of the first diode with a threshold potential, and determines that the switch has been operated if the anode potential of the first diode is lower than the threshold potential. Between the first diode and the second diode, the second diode is connected in parallel with the switch.
The present invention is characterized in that, in the above-described switch operation determining device, the determination circuit further comprises a second resistor and a third diode, which is connected in series with the second resistor, and the second resistor and the third diode are connected in parallel with the first resistor, the first diode, and the second diode, such that current flows to ground from the power source, a forward voltage of the third diode is set higher than a forward voltage of the first diode, and lower than a sum of the forward voltages of the first diode and the second diode, and the threshold potential is an anode potential of the third diode.
The present invention is characterized in that, in the above-described switch operation determining device, a rate of change of a temperature characteristic of the third diode resides within a range of a rate of change of the temperature characteristic of the first diode, and a rate of change of a temperature characteristic of the first diode and the second diode in total.
The present invention is characterized in that, in the above-described switch operation determining device, the forward voltage of the second diode is higher than the forward voltage of the first diode, and the second diode and the third diode are standard products the temperature characteristics of which are the same.
According to the present invention, the first diode and the second diode are connected in series, and the switch is connected in parallel with the second diode. Therefore, the anode potential of the first diode, which is used for determining operation of the switch, is not varied by the current value. As a result, there is no need to supply a large current, and energy consumption can be suppressed.
Further, since the switch and the second diode are connected in parallel, even though the resistance value of a leakage resistance varies, the potential of the anode side of the second diode does not vary so much whether a leak current occurs or does not occur, and thus setting of the threshold potential is easy.
According to the present invention, there is further provided a third diode, which is connected in parallel with the first diode and the second diode, a forward voltage of the third diode is set higher than a forward voltage of the first diode, and lower than a sum of the forward voltages of the first diode and the second diode, and the anode potential of the third diode is treated as the threshold potential. Therefore, the threshold potential itself can also be made to possess a temperature characteristic, and it is possible to widen the setting range of the threshold potential.
According to the present invention, since a rate of change of a temperature characteristic of the third diode resides within a range of a rate of change of the temperature characteristic of the first diode, and a rate of change of a temperature characteristic of the first diode and the second diode in total, the setting range of the threshold potential can further be widened.
According to the present invention, the forward voltage of the second diode is higher than the forward voltage of the first diode, and the second diode and the third diode are standard products the temperature characteristics of which are the same. Thus, it is possible to reduce the number of steps required for parts management as well as the costs. Further, the rate of change of the temperature characteristic of the third diode is easily set to reside within the range between the rate of change of the temperature characteristic of the first diode, and the rate of change of the temperature characteristic of the first diode and the second diode in total.